If one examines a typical symmetric multiprocessing computer system having a plurality of nodes interconnected through a given bus topology, it would typically be observed that when a request for a cache line is sent from one node to the next, through any of a number of protocols, the request would enter the remote pipeline at some point and, if necessary, generate a cross-invalidate request, during this pipe pass, to any of a number of processors on the remote node, as part of a process to obtain coherent ownership of the line for the requesting processor.
Normally, when a requester is trying to send a cross-invalidate request, for any of a given number of reasons, this cross-invalidate request can be rejected, which results in the requester having to make additional pipe passes to send this request, if it is necessary. This reject typically occurs as a result of bus conflicts, limitation in the processor as to how many cross-invalidate requests it can queue up (bias full), system throttling, or any of a number of other reasons that are typical to a multiprocessor system.
These conflicts result in additional delay in processing various operations and wasted pipe utilization/pipe passes as other operations could more efficiently utilize the system pipeline when compared to a request that is rejected for the primary operation that it was trying to accomplish during a given pipe pass.
Typically this type of behavior is considered a normal part of a multiprocessor system design and the impact that it has on the overall performance of the system is taken as a normal scaling effect in the system. As a result, little has been done to optimize this region, as the gains in additional processing power for each added processor in the system normally greatly exceed the system degradation that results from increased contention at the level of the shared cache(s).